•It is possible to determine the charge of an ion in an
ionic compound given the charges of the other ions
present in the compound.
•Determine the charge on the bromide ion in the
compound NaBr given that Na+ has a 1+ charge.
•The total charge is 0, so Br- must have a charge of 1in order to balance the 1+ charge of Na+.
•Numbers called oxidation numbers can be assigned
to atoms in order to keep track of electron
distributions in molecular as well as ionic
compounds.
Oxidation Numbers
•The charges on the ions in an ionic compound
reflect the electron distribution of the compound.
•In order to indicate the general distribution of
electrons among the bonded atoms in a molecular
compound or a polyatomic ion, oxidation numbers
are assigned to the atoms composing the compound
or ion.
•Unlike ionic charges, oxidation numbers do not
have an exact physical meaning; rather, they serve
as useful “bookkeeping” devices to help keep track
of electrons.
Assigning Oxidation Numbers
•In general when assigning oxidation numbers,
shared electrons are assumed to “belong” to the
more electronegative atom in each bond.
Shared electrons “belong”
to chlorine.
•More specific rules are provided by the following
guidelines.
1. The atoms in a pure element have an oxidation
number of zero.
• Examples: all atoms in sodium, Na,
oxygen, O2,
phosphorus, P4, and
sulfur, S8, have oxidation numbers of zero.
2. The more electronegative element in a binary
compound is assigned a negative number equal
to the charge it would have as an anion.
Likewise for the less electronegative element.
3. Fluorine has an oxidation number of -1 in all of
its compounds because it is the most
electronegative element.
4. Oxygen usually has an oxidation number of -2.
• Exceptions:
• In peroxides, such as H2O2, oxygen’s oxidation
number is -1.
• In compounds with fluorine, such as OF2,
oxygen’s oxidation number is +2.
5. Hydrogen has an oxidation number of +1 in all
compounds containing elements that are more
electronegative than it.
• It has an oxidation number of -1 with metals.
more
electronegative
metal hydride
6. The algebraic sum of the oxidation numbers of
all atoms in a neutral compound equal to zero.
7. The algebraic sum of the oxidation numbers of
all atoms in a polyatomic ion is equal to the
charge on the ion.
8. Although rules 1 through 7 apply to covalently
bonded atoms, oxidation numbers can also be
applied to atoms in ionic compounds similarly.
Examples:
Assign oxidation numbers to each atom in the
following compounds or ions:
a. UF6
b. H2SO4
c. ClO3-
Solution (a).
Start by placing known oxidation numbers above
the appropriate elements. From the guidelines,
we know that fluorine always has an oxidation
number of -1.
-1
UF6
Multiply known oxidation numbers by the
appropriate number of atoms and place the totals
underneath the corresponding elements.
There are six fluorine atoms, 6 x -1 = -6.
-1
UF6
-6
The compound UF6 is molecular. According to the
guidelines, the sum of the oxidation numbers must
equal zero.
The total of positive oxidation numbers is therefore
+6.
-1
UF6
+6 -6
Divide the total calculated oxidation number by the
appropriate number of atoms. There is only one
uranium atom in the molecule, so it must have an
oxidation number of +6.
+6 -1
UF6
+6 -6
Solution (b).
H2SO4
Oxygen and sulfur are each more electronegative
than hydrogen, so hydrogen has an oxidation
number of +1.
Oxygen is not combined with a halogen, nor is H2SO4
a peroxide. Therefore the oxidation number of
oxygen is -2.
Place these known oxidation numbers above the
appropriate symbols.
Place the total of the oxidation numbers
underneath.
+1
-2
H2SO4
+2
-8
The sum of the oxidation numbers must equal zero,
and there is only one sulfur atom in each molecule
of H2SO4.
Because (+2) + (-8) = -6, the oxidation number of
each sulfur atom must be +6.
+1 +6 -2
H2SO4
+2 +6 -8
Solution (c).
ClO3To assign oxidation numbers to the elements in ClO3-,
proceed as in parts (a) and (b). Remember, however,
that the total of the oxidation numbers should equal
the overall charge of the anion, -1.
The oxidation numbers of a single oxygen atom in the
ion is -2. The total oxidation number due to the three
oxygen atoms is -6.
For the chlorate ion to have a 1- charge, chlorine must
be assigned an oxidation number of +5.
+5 -2
ClO3+5 -6
Assign oxidation numbers to each atom in the
following compounds or ions:
1.HCl
9.N2O5
2.CF4
10.GeCl2
3.PCl3
11.H2O
4.SO2
12.NO25.HNO3
13.SO426.KH
14.H2CO3
7.P4O10
15.CS2
8.HClO3